Lithium iron phosphate (LiFePO4) has an olivine crystal structure, and is one of the positive electrode active materials for lithium ion secondary battery that has been intensively studied in recent years. One of the critical drawbacks of using LiFePO4 as the positive electrode active material in the lithium ion secondary battery is that the conductivity of LiFePO4 is extremely low (10−9 S/cm), which is significantly lower than that of the other positive electrode active materials like LiMn2O4 or LiCoO2 (10−3˜10−4 S/cm). As a result, the lithium ions are impeded while entering or leaving the positive electrode, thereby affecting the high rate capability of the lithium ion secondary battery employing LiFePO4 as the positive electrode active material; causing it to perform comparatively worse than LiMn2O4 or LiCoO2. Approaches in the literature for solving the problems associated with low conductivity of LiFePO4 can be divided into the following three categories:
1. Enhancing the conductivity of LiFePO4 by coating a layer of carbon on the surface thereof [1. N. Ravet, J. B. Goodenough, S. Besner, M. Simouneau, P. Hovington and M. Armand, Proceedings of 196th ECS Meeting, Hawaii, 17-22 Oct. 1999; 2 N. Ravet, Y. Chouinard, J. F. Magnan, S. Besner, M. Gauthier, and M. Armand, J. power sources, 97-98, 503 (2001); 3. P. P. Prosini, D. Zane, M. Pasquali, Electrochim. Acta, 46, 3517 (2001); 4. H. Huang, S.-C. Yin, F. Nazar, Electrochem. Solid State Lett., 4, A170 (2001); 5. Z. Chen, J. R. Dahn, J. Electrochem. Soc., 149, A1189(2002)];
2. Enhancing the conductivity of LiFePO4 by doping other metals into LiFePO4 [1. S. Y. Chung, J. T. Bloking, and Y. M. Chiang, Nat. Mater., 1, 123(2002)];
3. Enhancing the high rate capability of LiFePO4 by using nano LiFePO4 [1. A. Yamada, S. C. Chung, and K. Hinokuma, J. Electrochem. Soc., 148, A224(2001); 2. P. P Prosini, M. Carewska, S. Scaccia, P. Wisniewski, S. Passerini, M. Pasquali, J. Electrochem. Soc., 149, 886(2002); 3. S. Franger, F. Le Cras, C. Bourbon, H. Rouault, Electrochem. Solid State Lett., 5, 231(2002)].
Right now, the commonly used commercial LiFePO4 has been subjected to the carbon coating treatment, and the particle size thereof ranges approximately between sub-micron and micron. Although the conductivity of LiFePO4 can be raised to between 10−2 and 10−3 S/cm after the carbon coating treatment, its high rate capability remains low for other reasons yet to be clarified. Research has continuously been conducted to explore electrode design method capable of solving the issue of high rate capability of LiFePO4.
US Patent publication number 2005/0233219A1 and 2005/0233220A1 disclosed lithium ion secondary batteries with high rate capability that use LiFePO4 as the major ingredient in its positive electrode active materials. In the former, LiFePO4 is doped with titanium, zirconium, niobium, aluminum, or magnesium, and in the latter, a layer of the positive electrode active material on a current collector has an area specific impedance (hereinafter abbreviated as ASI) that is less than 20 Ω-cm2. The contents of these two US patents are incorporated herein by references.